XXL ski gloves are designed for hand circumferences over 10.5 inches, measured around the widest part of your palm (excluding the thumb). If you're right at the 9.5–10.5 inch range of an XL, you might still fit that size, but if you're comfortably above 10.5 inches, XXL is the way to go. Sizing up ensures your circulation isn't restricted, which is one of the fastest ways to end up with cold hands on the mountain.
The biggest mistake large-handed skiers make is squeezing into a smaller size because XXL options are limited. Gloves that are too tight compress your insulation and cut off blood flow, leaving you colder than you'd be in a properly fitted pair with less insulation. You want a small air gap at the fingertips — that trapped warm air is part of how your gloves keep you warm.
If you plan to wear a liner glove underneath, definitely factor that in and try the XXL on with the liner system you'll use on the slopes.